The present invention relates to a system for performing communication between mobile units by radio frequency signal, and more particularly relates to improved constitution of optical feeder link transmitting radio-frequency-modulated optical signal for the disposition of a wireless base station in such a system.
FIG. 8 shows the constitution of a distant-site portable or private-branch telephone system which has been conceived as a development of a near-site portable telephone system. FIG. 4 shows the constitution of an improved distant-site portable or private-branch telephone system for reducing the cost of the providing of the wireless base stations of the system shown in FIG. 8. Shown in these drawings are a wire communication signal SO, an electric signal S1 for communication, a radio-frequency-modulated optical signal S2, a wireless signal RF for communication, a telephone exchange 1, a modem 2, an electricity-to-light and light-to-electricity converter 3, an optical fiber cable 4, a wireless communication antenna S, wireless base stations 6 and 6', a private branch exchange station 7, a conventional metal cable 8, and a portable telephone 9. In the system shown in FIG. 3, the radius of each of service zones is made small enough to make it possible to use a limited number of frequencies for communication between all the pairs of the zones. However, this results in making the number and cost of the wireless base stations 6 of the system very large and high. To avoid such a problem, the system shown in FIG. 4 has been proposed and studied as mentioned by Shibuya et al. in Technical Report RCS90-12, 1990, published by Radio Communication System Research Group of Institute of Electronics, Information & Communication Engineers of Japan. In the system shown in FIG. 4, the modems 2 are disposed together in the private branch exchange station 7, the radio-frequency-modulated optical signal S2 is transmitted between the exchange station and each wireless base station 6' through the optical fiber cable 4 to send information between the stations, and the wireless base station is made of only the electricity-to-light and light-to-electricity converter 3 and the wireless communication antenna 5, so that the constitution of the system is simplified. For that reason, it is very probable that the system will be a practical one in the future.
It is common that information is transmitted not only through a cable but also through a wireless means, apart from whether a specific communication system such as a mobile-unit communication system and a fixed-unit communication system is used or not. A communication system in which such transmission is performed not through the use of electromagnetic waves but through the use of a light beam has already been put into practical use, as mentioned by Sakanaka et al. in Report B-481 on Autumnal National Convention of Institute of Electronics, Information & Communication Engineers of Japan, 1989.
Whether a communication system functions properly or not depends on how an appropriate wirelessly-communicable area, which is hereinafter referred to as service area, can be created. If the constitution of the system shown in FIG. 4 is adopted to form a private branch wireless telephone system in a building, there is no big problem. However, if the wireless base station 6' is provided in the building in the case that the service area of the telephone system covers a wide field such as an athletic ground and a working site, which is located next to the building, there is a limit upon the extension of the service area. To avoid this problem, the other wireless base station 6' needs to be provided in the field. For that purpose, the optical fiber cable 4 needs to be laid. However, this is not convenient in the case that the temporary or provisional laying of a communication line, which seems to become a main form of utilization of space in the field, is enough. When the service area of the telephone system provided in the building is desired to be extended to a building next to the former, the laying of the optical fiber cable 4 for the extension is sometimes inconvenient to make the extension impossible. It is conceivable that to avoid the inconvenience of the laying of the optical fiber cable, a signal is transmitted by electromagnetic waves from inside the extended service area to the wireless base station 6' in the wide field. However, because of electromagnetic wave interference, the frequency of the waves cannot be made equal to that of those for communication between the wireless base station 6' and the mobile telephone 9, and a new frequency needs to be allocated to the former waves. In other words, there are many restrictions due to the constitution of the telephone system and the statute of electromagnetic waves, to make it impossible to extend the service area in an easy and simple manner. If light is used instead of the electromagnetic waves to transmit the signal, the restrictions due to the statute are eliminated. However, if the above-mentioned communication system in which the light beam is used is adopted for the transmission, a function in which information in a signal transmitted through a cable is converted into an electric signal in a base band through modulation, the electric signal is then converted into an optical signal and the optical signal is then transmitted, and another function in which the optical signal is received and converted into an electric signal in a base band through demodulation, the electric signal is then transmitted through a cable are required. For that reason, the adoption of the communication system does not result in making the extension of the service area simpler, easier and less expensive.